Magnetic disks and disk drives are employed for storing data in magnetizable form. Preferably, one or more disks are rotated on a central axis in combination with data transducing heads positioned in close proximity to the recording surfaces of the disks and moved generally radially with respect thereto. Magnetic disks are usually housed in a magnetic disk unit in a stationary state with a magnetic head having a specific load elastically in contact with and pressed against the surface of the disk. Data are written onto and read from a rapidly rotating recording disk by means of a magnetic head transducer assembly that flies closely over the surface of the disk. Preferably, each face of each disk will have its own independent head.
In a magnetic media, digital information (expressed as combinations of “0's” and “1's”) is written on tiny magnetic bits (which themselves are made up of many even smaller grains). When a bit is written, a magnetic field produced by the disc drive's head orients the bit's magnetization in a particular direction, corresponding to either a 0 or 1. The magnetism in the head in essence “flips” the magnetization in the bit between two stable orientations.
Magnetic thin-film media, wherein a fine grained polycrystalline magnetic alloy layer serves as the active recording medium layer, are generally classified as “longitudinal” or “perpendicular,” depending on the orientation of the magnetic domains of the grains of the magnetic material. In longitudinal media (also often referred as “conventional” media), the magnetization in the bits is flipped between lying parallel and anti-parallel to the direction in which the head is moving relative to the disc. In perpendicular media, the magnetization of the disc, instead of lying in the disc's plane as it does in longitudinal recording, stands on end perpendicular to the plane of the disc. The bits are then represented as regions of upward or downward directed magnetization (corresponding to the 1's and 0's of the digital data).
FIG. 1 shows a disk recording medium and a cross section of a disc showing the difference between longitudinal and perpendicular recording. Even though FIG. 1 shows one side of the non-magnetic disk, magnetic recording layers are sputter deposited on both sides of the non-magnetic aluminum substrate of FIG. 1. Also, even though FIG. 1 shows an aluminum substrate, other embodiments include a substrate made of glass, glass-ceramic, NiP/aluminum, metal alloys, plastic/polymer material, ceramic, glass-polymer, composite materials or other non-magnetic materials.
Efforts are continually being made to increase the areal recording density, i.e., the bit density, or bits/unit area, and signal-to-medium noise ratio (SMNR) of the magnetic media. To continue pushing areal densities and increase overall storage capacity, the data bits must be made smaller and put closer together. However, there are limits to how small the bits may be made. If the bit becomes too small, the magnetic energy holding the bit in place may become so small that thermal energy may cause it to demagnetize over time. This phenomenon is known as superparamagnetism. To avoid superparamagnetic effects, disc media manufacturers have been increasing the coercivity (the “field” required to write a bit) of the disc. However, the fields that can be applied are limited by the magnetic materials from which the head is made, and these limits are being approached.
The increasing demands for higher areal recording density impose increasingly greater demands on thin film magnetic recording media in terms of coercivity (Hc), remanent coercivity (Hcr), magnetic remanance (Mr), which is the magnetic moment per unit volume of ferromagnetic material, coercivity squareness (S*), SMNR, over-write (OW), bit-error rate (BER), decay rate (DR) and thermal stability of the media. These parameters are important to the recording performance and depend primarily on the microstructure of the materials of the media. For example, as the SMNR is reduced by decreasing the grain size or reducing exchange coupling between grains, it has been observed that the thermal stability of the media decreases.
Newer longitudinal recording methods could allow beyond 140 gigabits per square inch in density. A great challenge however is maintaining a strong signal for the bits recorded on the media. When the bit size is reduced, the signal is decreased, making the bits more difficult to detect, as well as more difficult to maintain stable after recording information.
One of the key challenges to extending magnetic recording technology beyond the currently achieved 35-100 Gbit/in2 areal densities is to improve the signal to noise ratio by media noise suppression. However, as the storage density of magnetic recording disks has increased, the product of Mr and the magnetic layer thickness t has decreased and Hcr of the magnetic layer has increased. This has led to a decrease in the ratio Mrt/Hcr. To achieve a reduction in Mrt, the thickness t of the magnetic layer has been reduced, but only to a limit because the magnetization in the layer becomes susceptible to thermal decay. This decay has been attributed to thermal activation of small magnetic grains (the super-paramagnetic effect). The thermal stability of a magnetic grain is to a large extent determined by KuV, where Ku is the magnetic anisotropy constant of the magnetic layer and V is the volume of the magnetic grain. As the magnetic layer thickness is decreased, V decreases. Thus, if the magnetic layer thickness is too thin, the stored magnetic information might no longer be stable at normal disk drive operating conditions.
The increase in Ku is limited to the point where the coercivity Hc, which is approximately equal to Ku/Mr, becomes too large to be written by a conventional recording head. On the other hand, a reduction in Mr of the magnetic layer for a fixed layer thickness is limited by the coercivity that can be written. Increasing V by increasing inter-granular exchange can also increase thermal stability. However, this approach could result in a reduction in the SMNR of the magnetic layer.
In order to squeeze as much digital information as possible on a recording disc medium there is a need to find a film structure, which can benefit the low noise feature of laminated medium, but has acceptable thermal stability. Anti-ferromagnetically coupled (AFC) dual-layer media having with a five element magnetic alloy and a low moment stabilizing layer have been unexpectedly found to improve magnetic recording performance with reduced grain size, enhanced grain segregation and maintained high magnetic saturation (Ms) and magneto-crystalline anisotropy.